High speed circuit breakers



July 17, 1956 Filed May ll, 1951 A. D. FERGUSON HIGH SPEED CIRCUITBREAKERS 4 Sheets-Sheet 1 l I l I I I I Inven tor": AlexanderD-Fer'guson,

His Attorney.

July 17, 1956 A. D. FERGUSON 2,755,355

HIGH SPEED CIRCUIT BREAKERS Filed May 11, 1951 4 Sheets-Sheet 2 InventorA! exam dew" D. T er g uson y His Attorney July 17, 1956 A. D. FERGUSON2,755,355

HIGH SPEED CIRCUIT BREAKERS Q Filed May 11, 1951 4 Sheets-Sheet 3Inventor: Akexamdev D. Ferguson,

by His Attorney.

July 17, 1956 A. D. FERGUSON 2,755,355

HIGH SPEED CIRCUIT BREAKERS Filed May 11. 1951 4 Sheets-Sheet 4 F1241gig Ai 10s Invent or Alexander- D. Ferguson,

byW

His Ab'brney.

United States Patent HIGH SPEED QIRQUIT BREAKERS Alexander D. Ferguson,London, England, assignor to General Electric Company, a corporation ofNew York Application May 11, 1951, Serial No. 225,639

9 Claims. (Cl. 206-102) This invention .relates to electric circuitbreakers and in particular to circuit breakers of the type employed inelectric traction equipments for interrupting the main motor circuits inresponse to fault conditions. The invention has for its object toprovide an electric circuit breakor which will operate to open thecontacts thereof with high speed in response to fault conditions butwill discriminate between fault conditions in the traction motor circuitand normal overload currents therein so as not to open said contacts inresponse to normal overload currents.

According to the present invention, an electric circuit breaker of thekind referred to is provided with a holding magnet adapted whenenergized to retain the contacts in the closed position, said holdingmagnet cooperating with an armature so as to retain the latter inposition holding the contacts closed against a pull-off spring or otherbiasing means, and the magnetic system comprising the magnet andarmature is provided with two controlling coils connected or adapted tobe connected in the main traction circuit at or electrically adjacentthe two ends, respectively, of said circuit, one of said coils, which isthat connected or adapted to be connected at the grounded end of thetraction motor circuit, being arranged to assist the main excitation ofthe holding magnet, whereas the other of said coils is arranged tooppose said main excitation. With this arrangement during normalconditions the two controlling windings act in opopsition to one anotherin the magnetic circuit and the contacts are retained in the closedposition. In the event of a ground fault occurring in the traction motorcircuit at any point between the two demagnetizing windings, thatcontrolling winding at the grounded end of the main circuit will beshort-circuited. The other controlling winding, which is still carryingthe traction circuit current, therefore acts in opposition to the mainexcitation so as to allow the armature and the contacts to move to theopen position. Conveniently the two controlling windings may be mountedon the two poles, respectively, of the holding magnet.

In practice the most common fault resulting in excessive current in thetraction motor circuits is flashover across a traction motor armature,which may be due, for example, to line surges, accumulation of brakeshoe dust, brushes jumping out of engagement with the commutator, ormoisture due to condensation. All such faults, which are in the firstinstance, or develop into, ground faults, will result in diversion ofcurrent from one part of the traction motor circuit to cause operationof the circuit breaker in the manner above described. On the other hand,excessive motor current due purely to overloading of the motors does notresult in different currents at the two ends of the circuit and thecircuit breaker remains closed; the motor may be protected against suchoverloading by the usual overcurrent relay operating in conjunction witha line switch provided in addition to the circuit breaker according tothe invention. The holding magnet may conveniently comprise two parallellimbs united by a yoke, and

the controlling windings may be located respectively on 2,755,355 l'atented July 17, 1956 2 said two limbs, preferably adjacent thearmature, that is to say at the ends of the limbs remote from the yoke.

According to a further feature of the invention, the circuit breaker isprovided with contact closing means which, after operation to close thecontacts and bring the armature into engagement with the holding magnet,are retracted or otherwise displaced from the return path of anactuating member for the contacts and said armature so that the latteris free to return upon demagnetization of the holding magnet. Thearrangement is employed in conjunction with a line switch and includescontrol circuits whereby in reclosing the circuit breaker providedaccording to the invention the line switch initially remains in the openposition and is only closed when the circuit breaker is closed. Thedesired result may conveniently be obtained by means of interlockcontacts operated by the circuit breaker contacts and by the actuatingmeans, which interlock contacts are connected in series with one anotheror otherwise arranged so that the contacts must be closed and theactuating means must be retracted or otherwise displaced as above setforth before the operating circuit for the line switch is completed. Ingeneral the actuating means will comprise a compressed air piston ascommonly employed for operating line switches and contactors in tractionsystems although it will be understood that the invention is not limitedin this respect.

According to a further feature of the invention, the holding magnet isprovided with means for bypassing magnetic flux from parts of the polepieces on which the controlling windings are mounted, said meansincluding a variable air gap whereby the value of fault current at whichthe armature is released may be adjusted at will.

The holding coil may be energized directly from the line or from anauxiliary supply source as may be expedient in any particularapplication of the invention.

According to a further feature of the invention, the circuit breakercomprises two pairs of cooperating contacts connected in series with oneanother and a pair of separate arc chutes associated respectively withsaid pairs of contacts. The are chutes may be of any desiredconstruction but preferably are each arranged in accordance with theinvention set forth and claimed in British Letters Patent No. 478,824whereby the circuit breaker comprises in effect four are chutesproviding, on interruption of the circuit, four arcs connected in serieswith one another.

According to another feature of the invention, the moving contact orcontacts of the circuit breaker are mounted at one end of a pivotedlever, the other end of which carries the armature cooperating with theholding magnet, and an operating piston or other means for the contactsare arranged to actuate said lever to close the contacts and after suchactuation to release the lever so that on demagnetization of the holdingmagnet the lever is free to move to the contact open position. Where thecircuit breaker comprises a pair of arc chutes and moving contacts suchas hereinbefore indicated, said lever is preferably formed with orcoupled with two contactcarrying arms and may include a further armadapted to be operated by said actuating means; said further arm may bepositioned for movement between the two are chutes.

A preferred embodiment of circuit breaker according to the inventionwill now be described by way of example with reference to theaccompanying drawings,

Fig. 1 being a side elevation, partly in section, of the circuitbreaker,

Fig. 2 being a rear elevation, partly in section, and

Fig. 3 being a sectional plan view.

Fig. 1 is taken on the line 1-1 of Fig. 3, Fig. 2 is taken on the lineIIII of Fig. 1, whilst Fig. 3 is taken on various horizontal planes ofFigs. 1 and 2 as will be apparent from the following description.

Fig. 4 is an electrical circuit diagram showing the method of connectionof the circuit breaker according to the invention in a simplifiedtraction system.

Referring to Figs. 1, 2 and 3, the various parts of the circuit breakerare mounted on a main framework comprising four vertical metal bars 1,2, 3 and 4 which are bolted on their upper ends to a supporting bracket5 and at their lower ends are bolted to faces on an operating aircylinder 6 of the circuit breaker. The upper bracket 5 is provided withperforated lugs '7 and the cylinder 6 is provided with perforated lugs8. By means of these lugs the circuit breaker is adapted to be mountedin place on the usual framework for the line switches and contacts ofthe electric traction equipment in which the circuit breaker isemployed.

The bars 1 and 2 support a fixed contact 9 with which cooperates amoving contact iii. An arc chute 11 is provided for controlling the arcdrawn upon opening of the contacts. The bars 3 and 4 similarly supportanother fixed contact, the cooperating moving contact appearing at inFig. 3 and the arc chute at 111. The are chutes may be of any suitableconstruction but are preferably of the character described and claimedin the complete specification of Letters Patent No. 478,824 aforesaid,to which attention is hereby directed. Each of the arc chutes isprovided with a blow-out magnet and a blowout coil, shown at 12 and 13for the arc chute 11 in Fig. l. The fixed contact 9 is secured to abracket 14 preferably removably, which bracket is secured to the bars 1and 2 by means of a cleat 15. The four bars are provided with coveringsof insulating material. The blowout magnet 12 is secured to the bars 1and 2 by means of a cleat 16. The fixed contact and blow-out magnetassociated with the arc chute 11 is supported in similar manner from thebars 3 and 4, the corresponding cleats being shown in Fig. 2 at 15 and16. One end of each blow-out coil is connected directly with the bracketsupporting the associated fixed contact as shown at 17 in the case ofthe blow-out coil 13. The other end 18 of the coil 13 is connected incircuit as will hereinafter be described. The corresponding end 18' ofthe other blowout coil is connected with a conductor 19 as willhereinafter appear in more detail. Arcing horns 20 and 2d are mounted bycleats 2i and 21' on the bars 1, 2, 3 and 4. The ends of the arcs aremoved by the blow-out magnets on to these arcing horns when the movingcon tacts move to the open position in line with the arcing horns.

The framework of the circuit breaker includes a pair of angle ironmembers 22 and 23 which are supported from the end bracket 5 and the aircylinder 6 by means of horizontal bars 24, 25, 26 and 27. A cast bracket21S is bolted to the angle iron members 22 and 23, this bracketincluding at its ends trunnions 29 and 30 receiving a horizontal spindle31. A contact carrying lever 32 is mounted on this spindle. This lever,which is conveniently a casting, comprises a main lever portionextending forwardly of the spindle 31 into the space between the arcchutes 11 and 11'. Conveniently as shown the arc chutes are providedwith individual inclosures includ ing insulating side partitions 33 and34 and 33 and 34'. The lever 32 comprises also a rearwardly extendingportion which carries in rear of the angle iron members 22 and 23 amagnetic armature 35. This armature cooperates with upper polar faces36, 37 of a U-shaped holding magnet supported by the frame members 26and 27 and the arrangement of which will hereinafter be described inmore detail. The contact carrying lever is formed adjacent the trunnions29 and 30 with forwardly projecting arms 38 and 39 which at their upperand forward ends are bifurcated and support respectively the movingcontacts 10 and 10 by means of pins 40 and 45). As will be seen themoving contacts are supported be tween the tines of the two arms by saidpivot pins and are urged upwardly by means of compression springs limbs69 and 7-6 of the magnet.

of which that associated with the contact 10 appears at 41 in Fig. 1.These compression springs abut between the contact members 16 and 10 andtail portions of the lever arms 38 and 38' so as to urge the contactmembers upwardly whereby to provide in accordance with common practice aknuckling action of the moving contacts on engagement of the latter withthe cooperating fixed contacts 9 and 9'. Preferably the moving contactfingers are provided with replaceable tips as illustrated, in accordancewith the common practice.

The moving contacts 10 and 10' are provided with rearwardly extendingtail portions, that of the contact 10 appearing at 42 in Figs. 1 and 3whilst that of the contact 10' is in Fig. 3 broken away in order to showmore clearly the arrangement of the end arm of the contact operatinglever. The tail portions are connected directly together by a flexiblecopper conductor 43. By this means a circuit proceeds from the end 18 ofthe blowout coil 13, through the latter to the fixed contact 9, themoving contact 10, the conductor 43, the moving contact it), theassociated fixed contact, and the second blowout coil to the end 18'(Fig. 2) of said second blow-out coil.

Strap conductors 44 and 44' are provided for connectin the cleats 21 and21' and thereby the arcing horns 2i and 20 with the bracket 28 andthereby directly with the associated moving contacts 10 and 10.

The forwardly extending arms 38 and 39 of the contact operating leverare provided with tail portions 45 (Fig. 1) adapted to cooperate withforward surfaces 46 (Fig. l) on the bracket 23 so as to limit thedownward movement of the contacts 10. Conveniently a horizontal bufierstrip 47 is interposed between the tail portions 45 and surfaces 46.

The rearward end of the lever 32 has pivotal connection at 43 with thelower end of a tension spring 49 which at its upper end is anchored to abar 50 which is pivotally supported in trunnions 51 mounted on the angleiron frame members 22 and 23, this spring thus urging the lever in thecounter-clockwise direction in Fig. l to the illustrated contact openposition.

A roller 52 is mounted in the forwardly extending end of the lever 32 bymeans of a pin 53. This roller is engaged by a horizontal upper surfaceof a tappet 54 which is carried by a vertical piston rod of the pistonworking within the air cylinder 6.

An electromagnetically operated control valve for the cylinder 6 isshown at 55. This valve is of the type commonly employed in electrictraction equipment arranged so that energization of an operating coilthereof will apply air pressure to the piston, whilst de-energization ofthe coil will vent the cylinder to atmosphere.

An interlock assembly 56 is also mounted on the air cylinder 6 foroperation by the piston. In the example shown the interlock contacts areof the kind comprising fixed butt contacts 57 and 53 (Figs. 1 and 3) andcooperating pivoted contact fingers 59 and 60 pivoted on an axis at 61.The contact fingers are provided with spring biasing means by which theyare urged toward the cooperating fixed contacts and are adapted to besupplied from the latter by a cam member 62 of insulating material whichis carried by a rocker arm 63 pivoted at 64 and having pin and slotengagement at 65 with an L-shaped operating member 66 which is carriedby the piston rod.

The holding magnet is provided with a pair of main exciting coils 67 and68 located respectively on the two These limbs are united by ahorizontal yoke 71 which is clamped between the angle members 72 and 73which are secured by cleats '74 and to the bars 26 and 27, these cleatsserving also to secure the lower ends of the angle members 22 and 23 tosaid bars. The limbs 69 and 74 are provided with main pole piecesextending toward one another and providing the surfaces 3d and 37hereinbefore referred to for cooperation with the armature 35. The limbsare also provided with auxiliary p ole pieces 76 and '77 which extendtoward one another but are separated from one another by an air gap 78.Controlling windings 79 and are mounted on the poles 69 and 70 above theauxiliary pole pieces 76 and Y7, these controlling windings beingconnected in circuit as will hereinafter be described. The controllingwinding 79 is provided with terminal ends fill and 82 whilst thecontrolling winding 3i? is provided with a te minal 83 (Fig. 3) at oneend and at its other endcomprises the conductor 19 hereinbefore referredto.

A cylindrical member 34 of steel or other magnetic material is locatedin the air gap 78. The member 84 is slidably located in a bore in aplate 85 which, together with a plate as, is clamped to the poles of themagnet. Conveniently these plates may be provided with recesses in whichthe poles register whereby to provide rigidity of the magnet structure.The member 84 is attached to a threaded rod it? which has screw threadedengagement in a bore in the plate 86 so that by rotation of the rod 87the position of the member within the air gap 73 may be adjusted axiallyand thereby the reluctance of said air gap is adjustable to determinethe setting of the circuit breaker as will hereinafter appear.

The connections of the coils are shown in the simplified circuit diagramof ig. 4, in which the current collector of the equipment is shown at11%, a traction motor armature and field winding a iill and 102respectively, the return conductor at 193 and variable series resistanceat it will be understood that the traction motor circuits although shownin simplified form in Fig. 4 may be of any desired kind such as provideresistance and series-parallel notching in the usual manner. The circuitproceeds from the collector tilt to the end 13 of the blow-out coil 13(Fig. 1), through said coil to the fixed contact 9 from which, ashereinbefore described, the circuit proceeds through the moving contactslit and to the fixed contact 9. This contact W is connected directlywith one end of the second blow-out coil 13' and thence through theconductor 19 to the controlling winding and through the terminal end 83to the traction motor circuits. in general the traction motor circuitswill include a line switch of the usual construction. the contacts ofwhich are shown at 105, controlled by overload relay, the operating coilof which appears at 1%. The traction motor circuits are connected withthe return conductor M3 through the controlling winding 7%. The holdingcoils 6'7 and 63 are connected for energization directly from thecurrent collector, usually through control switches which will beoperated only when the equipment is initially brought into operation orfor resetting after tripping. The resetting arrangement will includealso a circuit controlling the valve 55.

in Fig. 4 the operating coil for the line switch 105 is shown at ocrosand is connected in a circuit which includes two sets of interlockcontacts m7 and M8. The contacts lib? are operated by the movements ofthe piston and in Fig. 1 comprises the contacts 57 and 59. Theseinterlock contacts are arranged so as to close only when the piston isin the retracted position, that is to say in the condition shown in Fig.l. The contacts 108 are operated by the lever 32 and may be of similarconstruction to those shown at 56 in Fig. l, but with an operating armcorresponding with the arm 63 connected for operation by the lever 32.The interlock contacts 1% are arranged so as to be closed only when thecircuit breaker is closed. It will be understood that the circuit of theco l OCltlS will include in addition the usual overload relay contactsand resetting means as otherwise well known in the art.

in the operation of this arrangement, for closing the circuit breaker,the holding coils 6'7 and 68 will be energized and air pressure appliedto the piston. The latter will therefoe move upwardly in Fig. 1 andcause clockwise movement of the lever 32 so as to bring the contacts it)and 10' into engagement with the corresponding fixed contacts and closethe circuit through these contacts. At the same time the armature 35will be brought into contact with the pole surfaces 36 and 37 and airpressure is then removed from the piston by de-energization of theoperating coil of the air valve 55 so that the piston returns to itsillustrated position. So long as the piston is away from the illustratedretracted position of Fig. 1, the interlock contacts and 5'9 (M7, Fig.4) are opened so that the line switch contacts cannot be closed. Whenthe contacts 9, lid, 9', id have been closed, then the contacts closeand upon return of the piston to the retracted position the coil OCltlS(Fig. 4) can be energized resr' ing in closure of the line switch andthe m0- tor can be energized under control of the usual mastercontroller.

The coils 6? and 6% produce opposite polarity on the poles of theholding magnet, for example as illustrated by the arrows in Fig. 4. Thecoil Stl acts in opposition to the coil as whilst the coil 79 acts inthe same direcion as the coil 67, that is to say the coils S0 and '79respectively oppose and assist the main excitation of the holdingmagnet. During normal conditions the current flowing in the coils i9 and$0 is the same and therefore acts in opposition to one another on themagnetic circuit, neutralizing one another so that the holding windingsretain the armature 35 in the closed position indicated by broken linesin Fig. l and the contacts remain closed.

in the event of a ground fault occuring in the traction motor circuit atany point between the windings 8t? and 79, the controlling winding 79will be short-circuited to a degree dependent upon the severity of thefault. The full traction motor current, however, still flows through thecontrolling winding 86 which therefore overcomes the winding 63 toreduce the resultant magnetic flux passing through the armature 35 sothat the latter is released and the contacts opened.

On the other hand, as hereinbefore set forth, excessive motor cu willaffect both poles of the holding mag" net equally and the circuitbreaker will remain in the closed position. The overload protection isprovided in the usual way by the line switch What i claim is:

l. A circuit including a load device, a circuit breaker having astationary contact and a cooperating movable Contact connected incircuit with said load device comprir ing a holding magnet having a corewith a main winding thereon proviom main excitation thereto, an armaturemember connected to s id movable Contact and cooperating with said hemagnet thereby to hold said contacts closed, means arranged to bias saidarmature memher away from said holding magnet thereby to open said and apair of controlling windings on said hold- ;t core 1'" lively connectedin circuit with load device adjacent the ends thereof, one of saidcontrolling windings being connected to assist said main winding and theother of said controlling windings being connected to oppose said main.vinding, an overload line switch having contacts connected in circuitwith said load device, and men is connected to said circuit breaker andoperable thereby arranged to prevent closing of said line switchcontacts when said circuit breaker contacts are open.

2. A circuit including a load device, a circuit breaker having astationary contact and a cooperating movable contact connected incircuit with said load device and comprising a holding magnet having acore with a main winding thereon providing main excitation thereto, anarmature member connected to said movable contact and cooperating withsaid holding magnet thereby to hold said contacts closed, contactclosing means engaging said armature member and arranged to move thesame into engage .ment with said holding magnet thereby to close saidcontacts, said contact closing means being movable out of engagementwith said armature member whereby said armature may return to its openposition upon de-energization of said holding magnet, means arranged tobias said armature member away from said holding magnet thereby to opensaid contacts, and a pair of controlling windings on said holding magnetrespectively connected in the circuit of said load device adjacent theends thereof, one of said controlling windings being connected to assistsaid main winding and the other of said controlling windings beingconnected to oppose said main winding, an overload line switch havingcontacts connected in circuit with said load device and having anoperating coil, and a pair of interlock contacts respectively connectedin series with said line switch operating coil, one of said interlockcontacts being connected to said armature member and closed therebyresponsive to closing of said circuit breaker contacts, the other ofsaid interlock contacts being connected to said contact closing meansand closed thereby in response to said contact closing means moving outof engagement with said armature member whereby said line switchoperating coil is energized only when said circuit breaker contacts areclosed and said contact closing means is moved out of engagement withsaid armature member.

3. An electric circuit breaker having two pairs of contacts connected inseries, a pair of separate arc chutes associated respectively with saidpairs of contacts, each of said pairs of contacs comprising a stationarycontact and a cooperating movable contact, said pair of contacts beingadapted to be connected in series with a load device, a holding magnethaving a core with a main winding thereon providing main excitationthereto, an armature member connected to said movable contacts andcooperating with said holding magnet thereby to hold said contactsclosed, means arranged to bias said armature member away from saidholding magnet thereby to open said contacts, and a pair of controllingwindings on said holding magnet core adapted to be respectivelyconnected in the circuit of said load device adjacent the ends thereof,one of said controlling windings being arranged to assist said mainwinding and the other of said controlling windings being arranged tooppose said main winding.

4. An electric circuit breaker having a stationary contact and acooperating movable contact adapted to be connected in circuit with aload device and comprising a holding magnet having a core with a mainwinding thereon providing main excitation thereto, a lever memberpivoted intermediate the ends thereof having said movable contactmounted at one end thereof, an armature member mounted on the other endof said lever member and cooperating with said holding magnet corethereby to hold said contacts closed, means arranged to bias said levermember to hold said armature member away from said holding magnet corethereby to open said contacts, contact closing means engaging said levermember and arranged to actuate the same to move said armature memberinto engagement with said holding magnet core thereby to close saidcontacts, said contact closing means being movable out of engagementwith said lever member whereby the same may return to its initialposition upon demagnetization of said magnet, and a pair of controllingwindings on said holding magnet core adapted to be respectivelyconnected in the circuit of said load device adjacent the ends thereof,one of said controlling windings being arranged to assist said mainwinding and the other of said controlling windings being arranged tooppose said main winding.

5. An electric circuit breaker having a pair of contacts connected inseries, a pair of separate arc chutes respectively associated with saidpair of contacts, each of said contacts comprising a stationary contactand a cooperating movable contact, said contacts being adapted to beconnected in circuit with a load device, a holding magnet having a corewith a main winding thereon providing main excitation thereto, a levermember pivoted intermediate its ends, a pair of arms formed on one endof said lever member respectively carrying said movable contacts,another arm formed at the other end of said lever member, an armaturemember mounted on said other lever member arm and cooperating with saidholding magnet core thereby to hold said contacts closed, means arrangedto bias said other lever member arm to hold said armature member awayfrom said holding member core thereby to open said contacts, contactclosing means engagaing said other lever member arm and arranged toactuate the same to move said armature member into engagement with saidholding magnet core thereby to close said contacts, said contact closingmeans being movable out of engagement with said other lever member armwhereby said lever member may return to its initial position uponde-energization of said magnet, and a pair of controlling windings onsaid holding magnet core adapted to be respectively connected in thecircuit of said load device adjacent the ends thereof, one of saidcontrolling windings being arranged to assist said main winding and theother of said controlling windings being arranged to oppose said mainwinding.

6. An electric circuit breaker comprising a pair of contacts connectedin series, a pair of separate arc chutes associated respectively withsaid pair of contacts, each of said contacts comprising a stationarycontact and a cooperating movable contact, said contacts adapted to beconnected in circuit with a load device, a holding magnet having a corewith a main winding thereon providing main excitation thereto, a levermember pivoted intermediate its ends, a pair of arms formed at one endof said lever member and respectively having said movable contactsmounted thereon, said lever member having another arm formed at itsother end, an armature member mounted on said other arm and cooperatingwith said holding magnet core thereby to hold said contacts closed,means arranged to bias said other arm to hold said armature member awayfrom said holding magnet core thereby to open said contacts, contactclosing means engaging said other arm and arranged to actuate the sameto move said armature member into engagement with said holding magnetcore thereby to close said contacts, said contact closing means beingmovable out of engagement with said other arm whereby said lever membermay return to its initial position upon de-energization of said magnet,said other arm being arranged for movement between said are chutes, anda pair of controlling windings on said holding magnet adapted to berespectively connected in the circuit of said lead device adjacent theends thereof, one of said controlling windings being arranged to assistsaid main winding and the other of said controlling windings beingarranged to oppose said main winding.

7. A circuit including a load device, a circuit breaker having astationary contact and a cooperating movable contact connected incircuit with said load device and comprising a holding magnet having acore with a main winding thereon providing main excitation thereto, anarmature member connected to said movable contact and cooperating withsaid holding magnet thereby to hold said contacts closed, means arrangedto bias said armature away from said holding magnet thereby to open saidcontacts, a pair of controlling windings on said holding magnet corerespectively connected in the circuit of said load device adjacent theends thereof, one of said controlling windings being arranged to assistsaid main winding and the other of said controlling windings beingarranged to oppose said main winding.

8. A circuit including a power terminal and a ground return line, adirect current load device, a circuit breaker having a stationarycontact and a cooperating movable contact connected in circuit with saidload device and comprising a holding magnet having a core with a mainwinding thereon providing main excitation thereto, an armature memberconnected to said movable contact and cooperating with said holdingmagnet thereby to hold said contacts closed, means arranged to bias saidarmature away from said holding magnet thereby to open said contacts,and a pair of similar controlling windings on said holding magnet corerespectively connected in the circuit of said load device adjacent theends thereof, the first of said controlling windings being arranged toassist said main winding and the second of said controlling windingsbeing arranged to oppose said main winding, said first control windingbeing connected at the ground return line whereby a ground fault betweensaid controlling windings will be detected.

9. An electric circuit breaker having a stationary contact and acooperating movable contact adapted to be connected in circuit with aload device to protect the device from ground faults and comprising aholding magnet having a core with a main winding thereon providing anexcitation thereto, an armature member connected to said movable contactand cooperating with said holding magnet thereby to hold said contactsclosed, contact closing means engaging said armature member and arrangedto move the same into engagement with said holding magnet thereby toclose said contacts, said contact closing means being movable out ofengagement with said armature member whereby said armature member may bereturned to its open position upon de-energization of said holdingmagnet, means arranged to bias said armature member away from saidholding magnet thereby to open said contacts, a pair of controllingwindings on said holding magnet adapted to be respectively connected inthe circuit of said load device adjacent the ends thereof, one of saidcontrolling windings being arranged to assist said main winding and theother of said controlling windings being arranged to oppose said mainwinding, a pair of auxiliary pole pieces connected to said core betweensaid main winding and said control windings to provide an auxiliary highreluctance flux path therebetween, and a magnetic cylindrical memberadjustably positioned in the gap between said auxiliary pole pieces toprovide a control of the reluctance of said high reluctance path.

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